Recently, many attempts have been made to remove toxic agents from air by using filtration systems, so that the air thus purified can be supplied to the vehicle operator or to the operator's compartment.
Up to now, as a rule, the filtration system has been controlled based on the hydrocarbons odor.
Repeated attempts have been made to control the switching on and off by means of measuring devices according to the load of toxic substances.
The installation of measuring systems with simultaneous evaluation is extremely costly and is considered to be scarcely practicable for economic reasons.
On the other hand, sensors that in each instance display a specific level of sensitivity for a specific toxic component can be used as indicating devices.
However, the development of measuring devices or sensors so as to obtain reliable readings is extremely difficult.
If one is to install a measurement and control system within a motor vehicle and thus ensure that the applied air is always filtered if the indicators in the vehicle indicate toxic agents, the air that is supplied to the vehicle interior through the filter would soon indicate a positive value relative to its degree of purity, and the filter system would be switched off.
If, however, the outside air contains toxic agents, after a period of for instance one or two minutes the measuring devices would respond and indicate the presence of such agents.
This is to say, that there would have to be constant switching on and off, with the appropriate delay phases without any guarantee that a continuous supply of air is ensured when toxic substances can become a nuisance in the area outside the vehicle.
For this reason, the measurement systems are installed outside a vehicle.
Considerable difficulties are encountered when this is done; for example, in fog, frost, sunshine following frost, or heavy rain
the measuring probes or the sensors are prevented from providing accurate readings because of a large number of adversely-effective environmental influences.
The installation of measuring devices with pre-drying based on gel filters, as an example, or the incorporation of measuring apparatuses in the air-aspiration ducts for the supply of air in the air-conditioning shaft or the normal fresh-air shaft is subjected to the constant changes in humidity and constant temperature variations and in addition to this, when the vehicle is in heavy traffic there is an efflux of gases from the area of the engine, repeatedly in the case of motorcars and trucks, these gases having a content of toxic substances that is not usual as a triggering value for a CO or hydrocarbon sensor but which, on the other hand, will falsify the picture by virtue of the cross-sensitivity of all measurement systems.